Salty Matters

The Blog is written by me, John Warren. Once every three or four weeks or so I will post an article or two on an evaporite topic that has piqued my interest. On the Saltwork Publications webpage (under "the Works") there is a growing library of pdfs and epubs based on these blogs. These articles on the website have much higher resolution extractable graphics in than in the blog. There is also a link to this set of pdfs and epubs on the home page (www.saltworkconsultants.com).

Volatile evaporite interactions with magma Part 1 of 3: Indications of hydrated salts?

John Warren - Sunday, February 10, 2019

Introduction

Direct and indirect interactions between magma and evaporites at a regional scale are neither well documented, nor well understood. Mostly, this is because little or no salt remains once the high-temperature interactions have run their course; instead, there is a suite of indirect geochemical and indicator-mineral assemblages (Warren, 2016). Aside from the presence of what can be ambiguous indicator suites, some hard-rock geologists with a career working in igneous and metamorphic terranes may not be well versed in textures indicative of the former presence of sedimentary evaporites, nor their varying volatility, nor their meta-evaporitic and meta-igneous siblings.

The term pyrometasomatic encompasses some, but not all, of the types of salt-magma interaction and reactions that occur when evaporites and molten magmas of different types are nearby. Styles of evaporites interactions with magma are a spectrum, with two endmember situations; 1) orthomagmatic (salt-assimilative and internal to the magma), and 2) paramagmatic (salt-interactive and external to the magma). Both encompass outcomes that can include a variety of substantial ore deposits (Warren, 2016; Chapter 16). Only in situations where igneous sills and dykes have intruded salt masses, with contacts preserved, can direct effects of magma-salt interaction be documented. Even then, determining the timing of the evaporite igneous interaction can be problematic; one must ask if the chemistry and texture indicate, 1) syn-igneous emplacement, or 2) post-emplacement alteration and deeply circulating groundwater flushing, or 3) a combination.

Orthomagmatic and paramagmatic evaporite associations are distinct from occurrences of primary igneous/magmatic anhydrites, which precipitate from sulphate-saturated melts. Igneous anhydrite forms independently of any sedimentary evaporite assimilation, as seen, for example, in anhydrite crystals crystallised in trachyandesitic pumice erupted from El Chichón Volcano in 1982, or in dacitic pumices erupted from Mount Pinatubo in 1991 and in acidic lavas in the Yanacocha district of northern Peru (Luhr et al., 2008; Chambefort et al., 2008). These evaporite assimilations are also distinct from fumarolic anhydrite, which precipitates where groundwaters and sulphur-bearing magmatic fluids interact, as on Usu Volcano, Hokkaido, and many central American and Andean volcanoes such as El Laco (Zimbelman et al., 2005). Likewise, they are distinct from the anhydrite precipitates (white smokers) in and below submarine vents across numerous mid-oceanic ridges (Humphris et al., 1995). See Warren 2016 (Chapter 16) for more geological detail on these non-evaporite-igneous anhydrite occurrences.

Cooking with salt (thermal decomposition of hydrated versus non-hydrated salts)

Perhaps the most critical factor controlling the local intensity of magmatic interaction with an evaporitic country rock is whether or not the sedimentary evaporite assemblage, in proximity to an igneous heat source, contains abundant hydrated salts, such as gypsum, polyhalite or carnallite. Hydrated evaporite salts, when interacting with the igneous realm, are highly volatile and likely to decompose. They tend to release their water of crystallisation at temperatures many hundreds of degrees below the melting points of their anhydrous counterparts (Table 1).


In contrast, anhydrous salts, such as halite beds intruded by igneous dykes or sills, are much less reactive. At a local scale (measured in metres) with respect to an intrasalt-igneous interaction, there are a number of documented thermally-driven alteration styles, typically created by the intrusion of dolerite dykes and sills into cooler halite, or the outflow of extrusive igneous flows over cooler halite beds (Knipping and Herrmann, 1985; Knipping, 1989; Grishina et al, 1992, 1998; Gutsche, 1988; Steinmann et al., 1999; Wall et al., 2010). Hot igneous material interacts with somewhat cooler anhydrous salt masses, typically halite or anhydrite, to create narrow but distinct heat and mobile fluid-release envelopes(Figure 1), also reflected in the resulting recrystallised inclusion-modified salt textures (Figure 2).


Based on studies of inclusion chemistry and homogenization temperatures in fluid inclusions in bedded halite near intrusives, it seems that the extent of the influence of a dolerite sill or dyke in bedded salt is marked by fluid (brine)-inclusion migration. This is evidenced by the disappearance of chevron structures and consequent formation of clear (sparry) recrystallised halite, with a new set of higher-temperature brine inclusions located at intercrystal or polyhedral intersections. Such a migration envelope is documented in bedded Cambrian halites intruded by end-Permian dolerite dykes in the Tunguska region of Siberia (Figure 2; Grishina et al., 1992). There, as a rule of thumb, an alteration halo extends up to twice the thickness of the dolerite sill above the sill and almost the thickness of the sill below (Figure 1).

Four inclusion type associations were found in halite as a function of the ratio of the distance of the sample from the intrusion contact (d) to the thickness of the intrusion (h), i.e. d/h (Figure 2). Chevron structures with aqueous inclusions progressively disappear as d/h decreases; the disappearance of chevrons occurs at greater distances above than below the intrusive sill. At d/h < 5 above the sill, a low-density CO2 vapour phase appears in brine inclusions, at d/h < 2 H2S-bearing liquid-CO2 inclusions appear, sometimes associated with carbonaceous material and orthorhombic S8, and for d/h < 0.9, CaCl2, CaCl2.KCl and nCaCl2.n MgCl2 solids occur in association with free water and liquid CO2 inclusions, with H2S, SCO, and Sg. The d/h values marking the transitions outlined above are lower below the sills than above. The water content of the inclusions progressively decreases on approaching the sills, whereas their CO2 content and density increase. Carnallite, sylvite and calcium chloride can occur as solid inclusions in the two associations nearest to the sill for d/h<2. Carnallite and sylvite occur as daughter minerals in brine inclusions. The presence of carbon dioxide is taken to indicate fluid circulation and dissolution/recrystallisation phenomena induced by the basalt intrusions. The origin of carbon dioxide is likely related to carbonate dissolution during magmatism (see Salty Matters, Oct 31, 2016).


In some shallow locations, relatively rapid magma emplacement can lead to linear breakout trends outlined by phreatomagmatic or phreatic explosion craters. Such phreatic explosion craters have been imaged on the Tertiary seafloor horizons in parts of the North Sea (Figure 3; Wall et al., 2010). The dykes were emplaced into Paleozoic and Mesozoic sediments and have a common upper termination in Early Tertiary sediments. The dykes are part of the British Tertiary volcanic province emplaced some 58 Ma. These dykes are characterised by a narrow 0.5–2 km wide vertical disturbance of seismic reflections that have linear plan view geometry. Negative magnetic anomalies directly align with the vertical seismic disturbance zones and indicate the presence of underlying igneous material. Linear coalesced collapse craters are found above the dykes. The collapse craters formed above the dyke due to the release of volatiles at the dyke tip and resulting gaseous expansion and subsequent volume loss. According to Wall et al. (2010), the larger craters likely formed due to explosive phreatomagmatic interaction between magma and pore water. The linearly aligned collapse craters can be considered an Earth analogue to Martian pit chain craters.

A phreatic eruption, also called a phreatic explosion, ultravulcanian eruption or steam-blast eruption, occurs when magma heats ground or surface water and is a separate but related occurrence to a phreatomagmatic eruption. A phreatomagmatic deposit typically contains solid inclusions of magmatic (igneous) material, whereas debris tied to a phreatic deposit does not, but ties to the effects of juvenile and deeply circulated hydrothermal waters. Extreme temperatures associated with an emplaced magma (anywhere from 500 to 1,170 °C) can cause a near-instantaneous phase change to steam, so forming a phreatomagmatic deposit. That is, rapid heating results in an intense explosion made up of steam, water, ash, rock, and volcanic bombs. During the eruption of Mount St. Helens, hundreds of steam explosions preceded the1980 Plinian eruption of the volcano core. Many authors argue a less intense geothermal event results in a mud volcano, but there are many other active mud volcanoes worldwide that tie to compactional overpressure unrelated to any magma emplacement (Warren et al., 2011). As the published interpretation of aligned phreatic breakout structures illustrated in Figure 3 is based on seismic without well control, the explosion mechanism may be solely phreatic heating or phreatomagmatic.


Deposits of phreatic eruptions (as contrasted with a phreatomagmatic eruption) typically include steam and rock fragments without the inclusion of fragments derived from liquid magma, lava or volcanic ash. The temperature of the phreatic fragments can range from cool to incandescent. So if molten magma is present, the resulting explosive debris deposit is typically classified as a phreatomagmatic eruption. These eruptions can create broad, low-relief craters called maars. In contrast, phreatic explosions lack debris derived from molten (igneous) material, but emplacement can be accompanied by carbon dioxide or hydrogen sulfide gas emissions. CO2 can asphyxiate at sufficient concentration; H2S is a broad spectrum poison. A 1979 phreatic eruption on the island of Java killed 140 people, most of whom were overcome by poisonous gases. Phreatic eruptions, even if the deposit lacks igneous rock fragments, are typically classed as a type of volcanic eruptions because a phreatic eruption can force juvenile fluids to the surface. But when a phreatic explosion is related to an igneous feature intersecting an evaporite bed, the resultant textures show a contrast between heating of anhydrous and hydrous salts


Hydrous salt interactions in Germany

Textures created by an igneous intrusion into a variably-hydrated evaporite succession can be studied in the dyke-and sill-intruded halite levels exposed in the walls of potash mines of the Werra-Fulda district of Germany (Figure 4; Steinmann et al., 1999; Schofield et al., 2014). There, the Permian Zechstein salt series contains two important potash salt horizons (2-10m thick), which are mined at a depths ≈ 800 m, from within a 400m thick halite host (Figure 4a). In the later Tertiary, basaltic melts intruded these Zechstein evaporites, but it seems only a few dykes reached the Miocene landsurface. The basaltic melt ties to regional volcanic activity, some 10 to 25 Ma. Basalts exposed in the halite-dominant portions of the mine walls are typically subvertical dykes, rather than sills. The basaltic intervals intersect the salt over zones up to several kilometres wide (Figure 4b). However, correlations of individual dyke swarms, either between different mines, or between surface and subsurface outcrops is difficult.


From a paleogeographic perspective, the Werra-Fulda Basin is situated in a southern embayment of the European Zechstein Basin. It contains cyclic evaporites of the Werra Formation (Z1). In the Neuhof area, the evaporites of the Zechstein are underlain by siliciclastic rocks of the Permian Rotliegend interval. The higher Zechstein-cycles (Z2 – Z7), on top of the Werra Formation, consist of a siliciclastic succession with intercalated limestone and anhydrite layers (Strauch et al., 2018; Beer and Barnasch, 2018). The Werra Formation is dominated by rock salt with a thickness up to 300 m.

Two potash seams (Seam Hessen and Seam Thüringen) separate the rock salt of the Werra Formation into three distinct units (Figure 4b). Lower, Middle and Upper Werra rock salt). Seam Hessen mainly consists of hard salt (kieserite, sylvite, halite and anhydrite). It is overlain by several, potash mineral-bearing horizons which show a strong vertical and lateral heterogeneity and consist of kieserite, sylvite, carnallite, halite and anhydrite. Internally, three separate units are identified within the potash Seam Hessen (Figure 4). The “Wurmsalz”, a hard salt with up to four strongly folded anhydritic clay bands represents the lower part of Seam Hessen. The middle part consists of massive, kieserite-rich hard salt with abundant sylvite lenses (“Flockensalz”). The “Bändersalz”, a banded hard salt which is typically intercalated with brownish, halitic layers occurs in the upper part of Seam Hessen. Potash Seam Thüringen usually occurs around 50 m below Seam Hessen. Its lower part is dominated by a well-bedded hard salt with intercalated rock salt. Its upper part consists of a variety of rock types including carnallite, sylvite and hard salt.

In the Fulda region the thermally-driven release of water of crystallisation within particular Zechstein salt beds intersecting igneous dykes creates thixotropic or subsurface “peperite” textures in hydrated carnallitite ore layers, where heated water of crystallisation escaped from the hydrated-salt lattice. Dehydration-driven loss of mechanical strength focuses zones of magma entry into particular horizons in the salt mass, wherever hydrated salt layers were intersected (Figure 5b verses 5c). In contrast, dyke and sill margins are much sharper and narrower in zones of contact with anhydrous salt intervals (Figure 5a, d; Schofield et al., 2014).

Accordingly, away from the immediate vicinity of the direct thermal aureole, heated and overpressured dehydration waters can enter a former Zechstein carnallite halite bed, and drive the creation of extensive soft sediment deformation and peperite textures in the previous hydrated layer (Figure 5c, d). Mineralogically, sylvite and coarse recrystallised halite dominate the salt fraction in the peperite intervals/beds. These deformed beds formed within a hydrated salt bed and so differ from the conventional notion of volcanic peperites indicating water-saturated sediment interactions with very shallow dyke or sill emplacements.

Sylvite in these altered zones is a form of dehydrated carnallite, not a primary-textured salt. In the Fulda region, such altered zones and deformed units can extend along former carnallite layers to tens or even a hundred or more metres from the dyke feeder. Ultimately, the deformed potash bed passes laterally out into the unaltered bed, which can retain abundant inclusion-rich primary chevron halite and carnallite (Figure 5c versus 5d). That is, nearer the basalt dyke, the carnallite is transformed mainly into inclusion-poor halite and sylvite, the result of recrystallisation combined with incongruent flushing of warm saline fluids mobilised from the hydrated carnallite crystal lattice as it was heated and decomposed in response to nearby dyke emplacement. During such Miocene salt alteration/thermal metamorphism in the Fulda region, NaCl-rich diagenetic and juvenile fluids were mixed with fluids originating from thermally-mobilised crystallisation water in the carnallite as it converted to sylvite.

Nearer the basalt dyke, the carnallite is largely transformed into inclusion-poor halite and sylvite, the result of incongruent flushing of warm saline fluids mobilised from the hydrated carnallite crystal lattice as it was heated by dyke emplacement. During Miocene salt alteration/thermal metamorphism in the Fulda region, NaCl-fluids were mixed with fluids originating from thermally-mobilised crystallisation water in the carnallite, as it converted to sylvite. This brine mixture altered the basalts during post-intrusive cooling, an event which numerical models suggest was quite rapid (Knipping, 1989): a dyke of less than 0.5 m thickness probably cooled to temperatures less than 200°C within 14 days of dyke emplacement.

Worldwide, igneous dykes intersecting salt beds tend to widen to become sills in two zones: 1) along evaporite units within the halite mass that contain hydrated salts, such as carnallite or gypsum (Figure 5b, c) and, 2) where rising magma has ponded and so created laccoliths at the upper or lower halite contact with the adjacent nonsalt strata, or against a salt wall (Warren, 2016). The first alteration of the hydrated salt layer is a form of mineral alteration and recrystallisation in response to a pulse of released water/steam as dyke-driven heating forces the dehydration of hydrated salt layers. The second alteration is often folding and fluid-like disaggregation of the former, now dehydrated, layer in response to the mechanical strength contrast at a hydrated-nonhydrated salt-bed contact (Warren, 2016).

Surface expression of hydrated bedded salts interacting with magma in Dallol, Ethiopia

Local potash ores typify thermal sump depressions in the Dallol and Musley areas (Figure 6a, b, c, 7) where a similar set of subsurface destabilisation processes occurred when rising magma reached the levels of hydrated salts (kainite and carnallite beds) in the Houston Formation of the Danakhil depression fill (see Warren 2016 and Bastow et al. 2018 for more detailed discussion of the potash stratigraphy). To attain these hydrated salt levels the rising dyke swarm had passed relatively passively through the Lower Rocksalt Formation (Salty Matters, April 29, 2015). Emplacement of the magma/dykes into hydrated evaporites below the vicinity of what is now the Dallol Mound would have mobilised and deformed the hydrated potash salt level, converting carnallite to sylvite, kainite to bischofite and lesser kieserite, as well as creating widespread cavities filled with rising pressured volatiles carried by MgCl and KCl brines. Pressurisacreation of a cavernous network filled with volatiles at the level of the Houston Formation would have aided in forming the four-way dip closure now seen on the exposed and eroding salt beds that make up much of the Dallol Mound surface.


Once these hydrothermal cavities dissolved and breached the way to surface, the feeder brines cool and precipitate prograde salts such as halite, sylvite and bischofite. Such destabilisation has likely accommodated the emplacement of a basaltic sill at the level of the potash salts, in turn driving the uplift of the lake beds above this region outlined by the centripetal dips of the Dallol Mound. Mound-related uplift and hydrothermal activity then stimulate the formation of natural areas of ground collapse, sulphurous and acidic springs and fumaroles, along with the creation of water-filled chimneys and doline sags, filling with various hydrothermal salts, in the vicinity of the volcanic mound  (Figure 6).

That is this type of potash in the Dallol Mound region is hydrothermally reworked from the uplifted equivalents of the Houston Formation. Even today this hydrology is precipitating carnallitite (associated with bischofite and minor kieserite) in various hydrothermal brine pools atop and around the Dallol Mound, such as the carnallite-dominant Crescent deposit (Figure 7). These hydrothermal salts owe their origins to daylighting of pressurised fluid systems and cavities.


The last pressurised phreatic explosion crater formed in 1926. They were created by the volatile products of hydrated salt layers (Houston Fm) where these salts had come into contact with thermal aureoles or actual lithologies of newly emplaced dykes that had penetrated the underlying halite section. Volcanic rock fragments and other igneous debris have yet to make it to the surface in the Dallol Mound region, although active volcanic mounds and flows do cover the saltflat surface tens of kilometres to the south (Erte Alle ) and north. Based on the analogy exposed within the Zechstein-hosted potash mines of the Fulda region of Germany, it is likely that as well as creating at-surface brine pools, this hydrothermal dyke-related hydrology locally converts most subsurface carnallitite to a disturbed sylvinite bed at the level of contact with the Houston Fm.

Implications

It seems a "one-size-fits-all" model does not characterise magmatic interactions with massively bedded evaporites. Instead, there is a mineralogical control to the intensity of the interaction and the depth of thermal influence of recrystallisation and mobilisation textures. When a dyke-swarm intersects halite or anhydrite the thermally-driven recrystallisation and fluid migration halo is more limited, as outlined in Figure 1 and Figure 5a, d.

In contrast, when a dyke swarm intersects an interval containing hydrous salts such kainite, carnallite or gypsum, the heating drives the expulsion of the bound-water at decomposition temperatures much lower than the salts melting point (Table 1). Such hydrous-salt intervals devolatise, fluidise and flow, with the effects of the heating halo extending much further away from the heat source, driven in part by steam-driven hydrofracturing. On cooling, the resulting mineralogy in the highly-deformed bed is dominated by the anhydrous form of the devolatised salt, as in the sylvite unit after carnallite as seen in potash seams adjacent to dykes in the Fulda Region (Figures 5b, c).

Closer in to the heat source, the basalt that has moved in along the hydrous potash beds show abundant peperite textures (Figure 5c; Schofield et al., 2014). Actually, this is a unique form of peperite that is tied to beds of hydrous evaporite. It forms outside the usual scenario envisaged for peperite whereby molten igneous material interacts with wet sediment, with the water in the wet sediment held in interparticle pores.

The classic definition of a peperite is that it is a "genetic term applied to a rock formed essentially in situ by disintegration of magma intruding and mingling with unconsolidated or poorly consolidated, typically wet sediments. The term also refers to similar mixtures generated by the same processes operating at the contacts of lavas and other hot volcaniclastic deposits with such sediments" (Skilling et al. 2002).

In the case of the bedded hydrous salt intervals, before the intrusion of the igneous heat source, there was little to no free water, other than occasional brine inclusions in associated halite chevrons. What makes these hydrous-salt peperites interesting is that it is the igneous heating drives a mineralogic transformation in the hydrous salts that makes the formerly "dry" salt bed become "wet" sediment.

Before our work in the Fulda region (Schofield et al., 2014), the nature of igneous interactions with evaporites was understood to be mainly that documented by studies in areas with intrusives interacting with thick anhydrous halite and anhydrite beds. The heating haloes were seen as driving recrystallisation and brine migration over limited lateral distances of a few metres. However, the potash seam interactions in the Fulda region show this alteration distance can be much greater (hundreds of metres) id hydrous salt layers are heated.

The surface geology in the Dallol Mound region of Ethiopia shows an even more impressive set of igneous dyke hydrated salt interactions (Warren, 2016). There the potash interval known as the Houston Formation is a tens-of-metres thick section of hydrated salts below the upper halite unit and atop the lower halite. When the rising igneous dyke swarm rose to the level of Houston Formation, it drove a broad linear devolatisation zone in the dyke-heated alteration halo. This, in turn, forced the formation of the closed anticlinal uplift structure that is the Dallol mound. The release of MgCl2 during volatisation also explains phreatic breakout features that are outlined by at-surface collapse dolines with their hot (104-108°C) brine lakes and unusual bischofite (MgCl2) precipitates. Likewise, the same set of processes explains the occurrences of metres to tens of metres thick bischofite intervals that are intersected in cores in some of the potash exploration wells in the vicinity of Dallol Mound (pers. obs). These are likely cavity fill deposits formed as a byproduct of kainite and carnallite devolatisation sourced at the level of Houston Formation.

This set of more mobile brine fluid escape features has implications for nuclear waste storage in halite successions where a storage cavity may be in proximity to an interval of hydrous evaporite salts. Halite-hosted purpose-built caverns in thick evaporite intervals are one of the safest places in the world to store waste but perhaps not in parts of the salt succession that entrain beds of hydrous salts such as carnallite or kainite (Warren, 2017).

References

Bastow, I. D., A. D. Booth, G. Corti, D. Keir, C. Magee, C. A.-L. Jackson, J. Warren, J. Wilkinson, and M. Lascialfari, 2018, The development of late-stage continental breakup: Seismic reflection and borehole evidence from the Danakil Depression, Ethiopia: Tectonics, v. 37.

Beer, W., and L. Barnasch, in press, Werra-Fulda-Becken, SDGG- Monography.

Chambefort, I., J. H. Dilles, and A. J. R. Kent, 2008, Anhydrite-bearing andesite and dacite as a source for sulfur in magmatic-hydrothermal mineral deposits: Geology, v. 36, p. 719-722.

Grishina, S., J. Dubessy, A. Kontorovich, and J. Pironon, 1992, Inclusions in salt beds resulting from thermal metamorphism by dolerite sills (eastern Siberia, Russia): European Journal of Mineralogy, v. 4, p. 1187-1202.

Grishina, S., J. Pironon, M. Mazurov, S. Goryainov, A. Pustilnikov, G. Fonderflaas, and A. Guerci, 1998, Organic inclusions in salt - Part 3 - Oil and gas inclusions in Cambrian evaporite deposits from east Siberia - A contribution to the understanding of nitrogen generation in evaporite: Organic Geochemistry, v. 28, p. 297-310.

Gutsche, A., 1988, Mineralreaktionen und Stotransporte an einem Kontakt Basalt-Hartsalz in der Werra-Folge des Werkes Hattorf: Unpubl. diploma thesis, thesis, Georg-August-Universita, Gottingen.

Humphris, S. E., P. M. Herzig, D. J. Miller, J. C. Alt, K. Becker, D. Brown, G. Brugmann, H. Chiba, Y. Fouquet, J. B. Gemmell, G. G., M. D. Hannington, N. G. Holm, J. J. Honnorez, G. J. Iturrino, R. Knott, R. Ludwig, K. Nakamura, S. Petersen, A. L. Reysenbach, P. A. Rona, S. Smith, A. A. Sturz, M. K. Tivey, and X. Zhao, 1995, The internal structure of an active sea-floor massive sulphide deposit: Nature, v. 377, p. 713-716.

Knipping, B., 1989, Basalt intrusions in evaporites: Lecture Notes in Earth Sciences (Springer-Verlag), v. 24, p. 132 pp.

Knipping, B., and A. G. Hermann, 1985, Mineralreaktionen und Stoff transporte an einem Kontakt Basalt-Carnallitit im Kalisalzhorizont Thüringen der Werra-Serie des Zechsteins: Kali und Steinsalz, v. 9, p. 111-124.

Luhr, J. F., 2008, Primary igneous anhydrite: Progress since its recognition in the 1982 El ChichÛn trachyandesite: Journal of Volcanology and Geothermal Research, v. 175, p. 394-407.

Schofield, N., I. Alsop, J. Warren, J. R. Underhill, R. Lehné, W. Beer, and V. Lukas, 2014, Mobilizing salt: Magma-salt interactions: Geology, v. 42, p. 599-602.

Skilling, I. P., J. D. L. White, and J. McPhie, 2002, Peperite: a review of magma–sediment mingling: Journal of Volcanology and Geothermal Research, v. 114, p. 1-17.

Steinmann, M., P. Stille, W. Bernotat, and B. Knipping, 1999, The corrosion of basaltic dykes in evaporites: Ar-Sr-Nd isotope and rare earth elements evidence: Chemical Geology, v. 153, p. 259-279.

Strauch, B., M. Zimmer, A. Zirkler, S. Höntzsch, and A. M. Schleicher, 2018, The influence of gas and humidity on the mineralogy of various salt compositions – implications for natural and technical caverns: Advances in Geoscience, v. 45, p. 227-233.

Wall, M., J. Cartwright, R. Davies, and A. McGrandle, 2010, 3D seismic imaging of a Tertiary Dyke Swarm in the Southern North Sea, UK: Basin Research, v. 22, p. 181-194.

Warren, J. K., 2016, Evaporites: A compendium (ISBN 978-3-319-13511-3): Berlin, Springer, 1854 p.

Warren, J. K., 2017, Salt usually seals, but sometimes leaks: Implications for mine and cavern stabilities in the short and long term: Earth-Science Reviews, v. 165, p. 302-341.

Warren, J. K., A. Cheung, and I. Cartwright, 2011, Organic Geochemical, Isotopic and Seismic Indicators of Fluid Flow in Pressurized Growth Anticlines and Mud Volcanoes in Modern Deepwater Slope and Rise Sediments of Offshore Brunei Darussalam; Implications for hydrocarbon exploration in other mud and salt diapir provinces (Chapter 10), in L. J. Wood, ed., Shale Tectonics, v. 93: Tulsa OK, AAPG Memoir 93 (Proceedings of Hedberg Conference), p. 163-196.

Zimbelman, D. R., R. O. Rye, and G. N. Breit, 2005, Origin of secondary sulfate minerals on active andesitic stratovolcanoes: Chemical Geology, v. 215, p. 37-60.

 

Brine evolution and origins of potash: primary or secondary? SOP in Quaternary saline lakes: Part 2 of 2

John Warren - Friday, November 30, 2018

 


Introduction

This, the second in this series of articles on potash brine evolution deals with production of sulphate of potash in plants that exploit saline hydrologies hosted in Quaternary saline sumps. There are two settings where significant volumes of sulphate of potash salts are economically produced at the current time; the Ogden salt pans on the northeast shore of the Great Salt Lake in Utah and Lop Nur in China. Although potassium sulphate salts precipitate if modern seawater is evaporated to the bittern stage, as yet there is no operational SOP plant utilising seawater. This is due to concurrent elevated levels of magnesium and chlorine in the bittern, a combination that favours the precipitation of carnallite concurrently with the precipitation of double sulphate salts, such as kainite (Figure 1). Until now, this makes the processing of the multi-mineralogic precipitate for a pure SOP product too expensive when utilising a marine brine feed.

 

Potash in Great Salt Lake, USA (SOP evolution with backreactions)

Today sulphate of potash fertiliser is produced via a combination of solar evaporation and brine processing, using current waters of the Great Salt Lake, Utah, as the brine feed into the Ogden salt pans, which are located at the northeastern end of the Great Salt Lake depression in Utah (Figure 2a). A simpler anthropogenic muriate of potash (MOP) brine evolution occurs in the nearby Wendover salt pans on the Bonneville salt flats. There, MOP precipitates as sylvinite in concentrator pans (after halite). The Bonneville region has a bittern hydrochemistry not unlike like the evolved Na-Cl brines of Salar de Atacama, as documented in the previous article, but it is a brinefield feed without the elevated levels of lithium seen in the Andean playa (Figure 3b).

Great Salt Lake brine contains abundant sulphate with levels sufficiently above calcium that sulphate continues to concentrate after most of the Ca has been used up in the precipitation of both aragonite and gypsum. Thus, as the brines in the anthropogenic pans at Ogden approach the bittern (post halite) stage, a series of sulphate double salts precipitate (Figure 4), along with carnallite and sylvite.


Great Salt Lake brines

The ionic proportions in the primary brine feed that is the endorheic Great Salt Lake water depends on a combination of; 1) the inflow volumes from three major rivers draining the ranges to the east, 2) groundwater inflow, 3) basin evaporation, and 4) precipitation (rainfall/snowfall) directly on the lake (Jones et al., 2009). Major solute inputs can be attributed to calcium bicarbonate-type river waters mixing with sodium chloride-type springs, which are in part hydrothermal and part peripheral recycling agents for NaCl held in the lake sediments. Spencer et al. (1985a) noted that prior to 1930, the lake concentration inversely tracked lake volume, which reflected climatic variation in the drainage. However, since that time, salt precipitation, primarily halite and mirabilite, and dissolution have periodically modified lake brine chemistry and led to density stratification and the formation of brine pockets of different composition.

Complicating these processes is repeated fractional crystallisation and re-solution (backreaction) of lake mineral precipitates. The construction of a railway causeway has restricted circulation, nearly isolating the northern from the southern part of the lake, which receives over 95% of the inflow. Given that Great Salt Lake waters are dominated by Na and Cl, this has led to halite precipitation in the north (Figures 2a, 3a; Gwynn, 2002). Widespread halite precipitation also occurred before 1959, especially in the southern area of the lake, associated with the most severe droughts (Jones et al., 2009; Spencer et al., 1985a). Spencer et al. (1985a) also described the presence of a sublacustrine ridge, which probably separated the lake into two basins at very low lake stands in the Quaternary. Fluctuating conditions emphasise brine differentiation, mixing, and fractional precipitation of salts as significant factors in solute evolution, especially as sinks for CaCO3, Mg, and K in the lake waters and sediments. The evolution of these brine/rock system depends on the concentration gradient and types of suspended and bottom clays, especially in relatively shallow systems.


Brine evolution across the Ogden pans

Figure 3a plots the known hydrochemistry of the inflow waters to the Great Salt Lake and their subsequent concentration. Evolving lake waters are always Na-Cl dominant, with sulphate in excess of magnesium in excess of potassium, throughout. Any post-halite evaporite minerals from this set of chemical proportions will contain post-halite potash bittern salts with elevated proportions of sulphate and magnesium and so will likely produce SOP rather than MOP associations. Contrast these hydrochemical proportions with the inflow and evolution chemistry in the pore brines of the Bonneville salt flat (Figure 3b) the Dead Sea and normal marine waters. Across all examples, sodium and chlorine are dominant and so halite will be the predominant salt deposited after aragonite and gypsum (Figure 4). Specifically, there are changes in sulphate levels with solar concentration (Figure 3b). In brines recovered from feeder wells in the Bonneville saltflat, unlike the nearby bajada well waters, the Bonneville salt flat brines show potassium in excess of sulphate and magnesium. In such a hydrochemical system, sylvite, as well as carnallite, are likely potassium bitterns in post-halite pans. The Wendover brine pans on the Bonneville saltflat produce MOP, not SOP, along with a MgCl2 brine, and have done so for more than 50 years (Bingham, 1980; Warren, 2016).


The mineral series in the Ogden pans

Figure 5 illustrates a laboratory-based construction of the idealised evolution of a Great Salt Lake feed brine as it passes through the various concentration pans. Figure 5 is a portion of the theoretical 25°C sulphate-potassium-magnesium phase diagram for the Great Salt Lake brine system and shows precipitates that are in equilibrium with brine at a particular concentration. Figures 4 and 5 represent typical brine concentration paths at summertime temperatures (Butts, 2002). Importantly, these figures do not describe the entire brine concentration story and local variations; mineralogical complexities in the predicted brine stream are related to thermal stratification, retention times and pond leakage. Effects on the chemistry of the brine due to the specific day-by-day and season-by-season variations of concentration and temperature, which arise in any solar ponding operation, require onsite monitoring and rectification. Such ongoing monitoring is of fundamental import when a pilot plant is constructed to test the reality of a future brine plant and its likely products.

 

Figure 6 illustrates the idealised phase evolution of pan brines at Ogden in terms of a K2SO4 phase diagram (no NaCl or KCl co-precipitates are shown; Felton et al., 2010). Great Salt Lake brine is pumped into the first set of solar ponds where evaporation initially proceeds along the line shown as Evap 1 until halite reaches saturation and is precipitated. Liquors discharged from the halite ponds are transferred to the potash precipitation ponds where solar evaporation continues as line Evap 2 on the phase diagram and potassium begins to reach saturation after about 75% of the water is removed. Potassium, sodium levels rise with further evaporation and schoenite precipitates in the schoenite crystalliser. After some schoenite precipitation occurs, the liquor continues to evaporate along the Evap 3 line to the point that schoenite, sylvite, and additional halite precipitate. Evaporation continues as shown by line Evap 3 to the point that kainite, sylvite and halite become saturated and precipitate. From this plot, the importance of the relative levels of extraction/precipitation of sulphate double salts versus chloride double salts is evident, as the evaporation plot point moves right with increasing chloride concentrations. That is, plot point follows the arrows from left to right as concentration of chloride (dominant ion in all pans) increases and moves the plot position right.


Production of SOP in the Great Salt Lake

To recover sulphate of potash commercially from pan bitterns fed from the waters of Great Salt Lake, the double salts kainite and schoenite are first precipitated and recovered in post-halite solar ponds (Figures 6, 7). The first salt to saturate and crystallise in the concentrator pans is halite. This is successively followed by epsomite, schoenite, kainite, carnallite, and finally bischofite. To produce a desirable SOP product requires ongoing in-pan monitoring and an on-site industrial plant whereby kainite is converted to schoenite. The complete salt evolution and processing plant outcome in the Ogden facility is multiproduct and can produce halite, salt cake and sulphate of potash and a MgCl2 brine product. Historically, sodium sulphate was recovered from the Great Salt Lake brines as a byproduct of the halite and potash production process, but ongoing low prices mean Na2SO4 has not been economically harvested for the last decade or so.

The complete production and processing procedure is as follows (Figure 7; Butts, 2002, 2007; Felton et al., 2010): 1) Brine is pumped from the Great Salt Lake into solar evaporation ponds where sodium chloride precipitates in the summer. 2) When winter weather cools the residual (post-halite) brine in the pans to -1 to -4°C, sodium sulphate crystals precipitate as mirabilite in a relatively pure state. Mirabilite crystals can be picked up by large earth-moving machinery and stored outdoors each winter until further processing takes place. 3) The harvested mirabilite can be added to hot water, and anhydrous sodium sulphate precipitated by the addition of sodium chloride to the heated mix to reduce sodium sulphate solubility through the common ion effect. The final salt cake product is 99.5% pure Na2SO4. 4) To produce SOP, Great Salt Lake brines are allowed to evaporate in a set of halite ponds, until approaching saturation with potassium salts. The residual brine is then transferred to a mixing pond, where it mixes with a second brine (from higher up the evaporation series, that contains a higher molar ratio of magnesium to potassium. 5) This adjusted brine is then allowed to evaporate to precipitate sodium chloride once more, until it is again saturated with respect to potassium salts. 6) The saturated brine is then transferred to another pond, is further evaporated and precipitates kainite (Figures 5, 6). Kainite precipitation continues until carnallite begins to form, at which time the brine is moved to another pond and is allowed to evaporate further to precipitate carnallite. 6) Some of the kainite-depleted brine is recycled to the downstream mixing pond to maintain the required molar ratio of magnesium to calcium in this earlier mixing pond (step 4). 7) Once carnallite has precipitated, the residual brine is transferred to deep storage and subjected to winter cooling to precipitate additional carnallite as it is a prograde salt. 8) Cryogenically precipitated carnallite can be processed to precipitate additional kainite by mixing it with a kainite-saturated brine. 9) MgCl2-rich end-brines in the post-carnallite bittern pans are then further processed to produce either MgCl2 flakes or a 32% MgCl2 brine. These end-bitterns are then used as a feedstock to make magnesium metal, bischofite flakes, dust suppressants, freeze preventers, fertiliser sprays, and used to refresh flush in ion exchange resins.

Some complexities in the observed mineral precipitation series in the Ogden Pans

Under natural solar pond conditions in the Ogden Pans, the brine temperature fluctuates with the air temperature across day-night and seasonal temperature cycles, and there is a lag time for temperature response in waters any brine pan, especially if the pan is heliothermic. Atmosphere-driven fluctuations in temperature results in changes in ion saturations, which can drive selective precipitation or dissolution of salts in the brine body. Air temperature in the Ogden pans may be 35°C during the day and 15°C at night. Brine at point A in figure 5 may favour the formation of kainite during the daytime and schoenite at night. The result of the diurnal temperature oscillation is a mixture of both salts in a single pond from the same brine. In terms of extracted product, this complicates ore processing as a single pan will contain both minerals, produced at the same curing stage, at the same time, yet one double salt entrains KCl, the other K2SO4, so additional processing is necessary to purify the product stream (Butts, 2002).

The sulphate ion in the pan waters is particularity temperature sensitive, and salts containing it in GSL pans tend to precipitate at cooler temperatures. Surficial cooling during the summer nights can cause prograde salts to precipitate, but the next day's heat generally provides sufficient activation energy to cause total dissolution of those salts precipitated just a few hours before (Butts, 2002). It is not unusual to find a 0.5 cm layer of hexahydrite (MgSO4.6H2O) at the bottom of a solar pond in the morning, but redissolved by late afternoon.

Under controlled laboratory conditions, brine collected from the hypersaline north arm of the Great Salt Lake will not crystallise mirabilite  until the brine temperature reaches 2°C or lower. Yet, in the anthropogenic solar ponds, mirabilite has been observed to crystallise at brine temperatures above 7°C. During the winter, as the surface temperature of the GSL pan brine at night becomes very cold (2°C or lower), especially on clear nights, and mirabilite rafts will form on and just below the brine surface and subsequently sink into the somewhat warmer brine at the floor of the pond. Because there is insufficient activation energy in this brine to completely redissolve the mirabilite, it remains on the pan floor, until warmer day/night temperatures are attained. However, it is also possible for salts precipitated by cooling to be later covered by salts precipitated by evaporation, which effectively prevents dissolution of those more temperature-sensitive salts that would otherwise redissolve (Butts, 2002).

There are also longer terms seasonal influences on mineralogy. Some salts deposited in June, July, and August (summer) will convert to other salts, with a possible total change in chemistry, when they are exposed to colder winter temperatures and rainfall. Kainite, for example, may convert to sylvite and epsomite and become a hardened mass on the pond floor; or if it is in contact with a sulphate-rich brine, it can convert to schoenite. Conversely, mirabilite will precipitate in the winter but redissolve during the hot summer months.

The depth of a solar pond also controls the size of the crystals produced. For example, if halite (NaCl) is precipitated in a GSL pond that is either less than 8 cm or more than 30cm deep, it will have a smaller crystal size than when precipitated in a pond between 8 and 30 cm deep. Smaller crystals of halite are undesirable in a de-icing product since a premium price is paid for larger crystals.

In terms of residence time, some salts require more time than others to crystallise in a pan. Brine that is not given sufficient time for crystallisation before it is moved into another pond, which contains brine at a different concentration, will produce a different suite of salts. For example, if a brine supersaturated in ions that will produce kainite, epsomite, and halite (reaction I), is transferred to another pond, the resulting brine mixture can favour carnallite (reaction 2), while kainite salts are eliminated.

Reaction 1: 9.75H2O + Na+ + 2Cl- + 2Mg2+ + K+ + 2SO42+ —> MgSO4.KCl +2.75H2O + MgSO4.7H2O + NaCl

Reaction 2: 12H2O + Na+ + 4Cl- + 2Mg2+ + K+ + 2SO42+ —> MgCl2.6H2O +MgSO4.6H2O + NaCl

Reaction 1 retains more magnesium as MgCl2 in the brine; reaction 2 retains more sulphate. In reaction 2, it is also interesting to note the effect of waters-of-hydration on crystallization; forcing out salts with high waters of crystallization results in higher rates of crystallization. The hydrated salts remove waters from the brine and further concentrate the brine in much the same way as does evaporation.

Pond leakage and brine capture (entrainment) in and below the pan floor are additional influences on mineralogy, regardless of brine depth or ponding area. As mentioned earlier, to precipitate bischofite and allow for MgCl2 manufacture, around ninety-eight percent of the water from present North Arm brine feed must evaporate. If pond leakage causes the level of the ponding area to drop too quickly, it becomes near impossible to reach saturation for bischofite (due to brine reflux). Control of pond leakage in the planning and construction phases is essential to assure that the precipitated salts contain the optimal quantity of the desired minerals for successful pond operation.

The opposite of leakage is brine retention in a precipitated layer; it can also alter brine chemistry and recovery economics. Brine entrained (or trapped) in the voids between salt crystals in the pond floor is effectively removed from salt production and so affects the chemistry of salts that will be precipitated as concentration proceeds and can also drive unwanted backreactions. The time required to evaporate nearly ninety percent of the water from the present north arm Great Salt Lake brine in the Ogden solar pond complex, under natural steady state conditions, is approximately eighteen months.

Summary of SOP production procedures in Great Salt Lake

Sulphate of potash cannot be obtained from the waters of the Great Salt Lake by simple solar evaporation (Behrens, 2002). As the lake water is evaporated, first halite precipitates in a relatively pure form and is harvested. By the time evaporative concentration has proceeded to the point that saturation in a potash-entraining salt occurs, most of the NaCl has precipitated. It does, however, continue to precipitate and becomes the primary contaminant in the potassium-bearing salt beds in the higher-end pans.

Brine phase chemistry from the point of potassium saturation in the evaporation series is complicated, and an array of potassium double salts are possible, depending on brine concentration, temperature and other factors. Among the variety of potash minerals precipitated in the potash harvester pans, the majority are double salts that contain atoms of both potassium and magnesium in the same molecule, They are dominated by kainite, schoenite, and carnallite. All are highly hydrated; that is, they contain high levels of water of crystallisation that must be removed during processing. SOP purification also involves removal of the considerable quantities of sodium chloride that are co-precipitated, after this the salts must be chemically converted into potassium sulphate.

Controlling the exact mineralogy of the precipitated salts and their composition mixtures is not possible in the pans, which are subject to the vagaries of climate and associated temperature variations. Many of the complex double salts precipitating in the pans are stable only under fixed physiochemical conditions, so that transitions of composition may take place in the ponds and even in the stockpile and early processing plant steps.

While weathering, draining, temperature and other factors can be controlled to a degree, it is essential that the Great Salt Lake plant be able to handle and effectively accommodate a widely variable feed mix (Behrens 2002). To do this, the plant operator has developed a basic process comprising a counter-current leach procedure for converting the potassium-bearing minerals through known mineral transition stages to a final potassium sulfate product (Figure 7). This set of processing steps is sensitive to sodium chloride content, so a supplemental flotation circuit is used to handle those harvested salts high in halite. It aims to remove the halite (in solution) and upgrade the feed stream to the point where it can be handled by the basic plant process.

Solids harvested from the potash ponds with elevated halite levels are treated with anionic flotation to remove remaining halite (Felton et al., 2010). To convert kainite into schoenite, it is necessary to mix the upgraded flotation product with a prepared brine. The conversion of schoenite to SOP at the Great Salt Lake plant requires that new MOP is added, over the amount produced from the lake brines. This additional MOP is purchased from the open market. The schoenite solids are mixed with potash in a draft tube baffle reactor to produce SOP and byproduct magnesium chloride.

The potassium sulfate processing stream defining the basic treatment process in the Great Salt Lake plant is summarised as Figure 7, whereby once obtaining the appropriate chemistry the SOP product is ultimately filtered, dried, sized and stored. Final SOP output may then be compacted, graded, and provided with additives as desired, then distributed in bulk or bagged, by rail or truck.


Lop Nur, Tarim Basin, China (SOP operation)

Sulphate of potash (SOP) via brine processing (solution mining) of lake sediments and subsequent solar concentration of brines is currently underway in the fault-bound Luobei Hollow region of the Lop Nur playa, in the southeastern part of Xinjiang Province, Western China (Liu et al., 2006; Sun et al., 2018). The recoverable sulphate of potash resource is estimated to be 36 million tonnes from lake brine (Dong et al., 2012). Lop Nur lies in the eastern part of the Taklimakan Desert (Figure 8a), China’s largest and driest desert, and is in the drainage sump of the basin, some 780 meters above sea level in a BSk climate belt. The Lop Nur depression first formed in the early Quaternary, due to the extensional collapse of the eastern Tarim Platform and is surrounded and typically in fault contact with the Kuruktagh (to the north), Bei Shan (to east) and Altun (to the south) mountains (Figure 8b).

The resulting Lop Nur (Lop Nor) sump is a large groundwater discharge playa that is the terminal point of China’s largest endorheic drainage system, the Tarim Basin, which occupies an area of more than 530,000 km2 (Ma et al., 2010). The Lop Nur sump is the hydrographic base level to local and regional groundwater and surface water flow systems, and thus collectively captures all river and subsurface flow originating in the surrounding mountainous regions. The area has been subject to ongoing Quaternary climate and water supply oscillations, which over the last few hundred years has driven concentric strandzone contractions on the playa surface, to form what is sometimes called the “Great Ear Lake" of the Lop Nur sump (Liu et al., 2016a).

Longer term widespread climate oscillations (thousands of years) drove precipitation of saline glauberite-polyhalite deposits, alternating with more humid lacustrine mudstones especially in fault defined grabens with the sump. For example, Liu et al. (2016b) conducted high-resolution multi-proxy analyses using materials from a well-dated pit section (YKD0301) in the centre of Lop Nur and south of the Luobei depression. They showed that Lop Nur experienced a progression through a brackish lake, saline lake, slightly brackish lake, saline lake, brackish lake, and playa in response to climatic changes over the past 9,000 years.

Presently, the Lop Nur playa lacks perennial long-term surface inflow and so is characterised by desiccated saline mudflats and polygonal salt crusts. The upward capillary flux from the shallow groundwater helps to maintain a high rate of evaporation in the depression and drives the formation of a metre-thick ephemeral halite crust that covers much of the depression (Liu et al., 2016a).

Historically, before construction of extensive irrigation systems in the upstream portion of the various riverine feeds to the depression and the diversion of water into the Tarim-Kongqi-Qargan canal, brackish floodwaters periodically accumulated in the Lop Nur depression. After the diversion of inflows, terminal desiccation led to the formation of the concentric shrinkage shorelines, that today outline the “Great Ear Lake” region of the Tarim Basin (Figure 8b; Huntington, 1907; Chao et al., 2009; Liu et al., 2016a).

The current climate is cool and extremely arid (Koeppen BSk); average annual rainfall is less than 20 mm and the average potential evaporation rates ≈3500 mm/yr (Ma et al., 2008, 2010). The mean annual air temperature is 11.6°C; higher temperatures occur during July (>40°C), and the lower temperatures occur during January (<20°C). Primary wind direction is northeast. The Lop Nor Basin experiences severe and frequent sandstorms; the region is well known for its wind-eroded features, including many layered yardangs along the northern, western and eastern margins of the Lop Nur salt plain (Lin et al., 2018).


Salinity and chemical composition of modern groundwater brine varies little in the ‘‘Great Ear” area and appears not to have changed significantly over the last decade (Ma et al., 2010). Dominant river inflows to the Lop Nor Basin are Na-Mg-Ca-SO4-Cl-HCO3 waters (Figure 9). In contrast, the sump region is characterised by highly concentrated groundwater brines (≈350 mg/l) that are rich in Na and Cl, poor in Ca and HCO3+CO3, and contain considerable amounts of Mg, SO4 and K, with pH ranging from 6.6 to 7.2 (Figure 9). When concentrated, the Luobei/Lop Nur pore brines is saturated with respect to halite, glauberite, thenardite, polyhalite and bloedite (Ma et al., 2010; Sun et al., 2018).

Groundwater brines, pooled in the northern sub-depression, mostly in the Luobei depression, are pumped into a series of pans to the immediate south, where sulphate of potash is produced via a set of solar concentrator pans. Brines in the Luobei depression and adjacent Xingqing and Tenglong platforms are similar in chemistry and salinity to the Great Ear Lake area but with a concentrated saline reserve due to the presence of a series of buried glauberite-rich beds (Figure 9; Hu and Wang, 2001; Ma et al., 2010; Sun et al., 2018).

K-rich mother brines in the Luobei hollow also contain significant MgSO4 levels and fill open phreatic pores in a widespread subsurface glauberite bed, with a potassium content of 1.4% (Liu et al., 2008; Sun et al., 2018). Feed brines are pumped from these evaporitic sediment hosts in the Luobei sump into a large field of concentrator pans to ultimately produce sulphate of potash (Figure 8a).

Brine chemical models, using current inflow water and groundwater brine chemistries and assuming open-system hydrology, show good agreement between theoretically predicted and observed minerals in upper parts of the Lop Nor Basin succession (Ma et al., 2010). However, such shallow sediment modelling does not explain the massive amounts of glauberite (Na2SO4.CaSO4) and polyhalite (K2SO4MgSO4.2CaSO4.2H2O) recovered in a 230 m deep core (ZK1200B well) from the Lop Nor Basin (Figure 9a).


Hydrochemical simulations assuming a closed system at depth and allowing brine reactions with previously formed minerals imply that widespread glauberite in the basin formed via back reactions between brine, gypsum and anhydrite and that polyhalite formed via a diagenetic reaction between brine and glauberite. Diagenetic textures related to recrystallisation and secondary replacement are seen in the ZK1200B core; they include gypsum-cored glauberite crystals and gypsum replacing glauberite. Such textures indicate significant mineral-brine interaction and backreaction during crystallisation of glauberite and polyhalite (Liu et al., 2008). Much of the glauberite dissolves to create characteristic mouldic porosity throughout the glauberite reservoir intervals (Figure 10, 11b)


Mineral assemblages predicted from the evaporation of Tarim river water match closely with natural assemblages and abundances and, in combination with a model that allows widespread backreactions, can explain the extensive glauberite deposits in the Lop Nor basin (Ma et al., 2008, 2010). It seems that the Tarim river inflows, not fault-controlled upwelling hydrothermal brines, were the dominant ion source throughout the lake history. The layered distribution of minerals in the more deeply cored sediments documents the evolving history of inflow water response to wet and dry periods in the Lop Nor basin. The occurrence of abundant glauberite and gypsum below 40 m depth, and the absence of halite, polyhalite and bloedite in the same sediment suggests that the brine underwent incomplete concentration in the wetter periods 10b).

In contrast, the increasing abundance of halite, polyhalite and bloedite in the top 40 m of core from the ZK1200B well indicate relatively dry periods (Figure 10a), where halite precipitated at lower evaporative concentrations (log Concentration factor = 3.15), while polyhalite and bloedite precipitated at higher evaporative concentrations (log = 3.31 and 3.48 respectively). Following deposition of the more saline minerals, the lake system once again became more humid in the later Holocene, until the anthropogenically-induced changes in the hydrology over the last few decades, driven by upstream water damming and extraction for agriculture (Ma et al., 2008). These changes have returned the sump hydrology to the more saline character that it had earlier in the Pleistocene.

The Lop Nur potash recovery plant/factory and pan system, located adjacent to the LuoBei depression (Figures 8, 11a), utilises a brine-well source aquifer where the potash brine is reservoired in intercrystalline and vuggy porosity in a thick stacked series of porous glauberite beds/aquifers.

Currently, 200 boreholes have been drilled in the Lop Nor brine field area showing the Late-Middle Pleistocene to Late Pleistocene strata are distributed as massive, continuous, thick layers of glauberite with well-developed intercrystal and mouldic porosity, forming storage space for potassium-rich brine (Figure 11b; Sun et al., 2018). However, buried faults and different rates of creation of fault-bound accommodation space, means there are differences in the brine storage capacity among the three brinefield extraction areas; termed the Luobei depression, the Xingqing platform and the Tenglong platform areas (Figures 9a, 11a).

In total, there are seven glauberitic brine beds defined by drill holes in the Luobei depression, including a phreatic aquifer, W1L, and six artesian aquifers, W2L, W3L, W4L, W5L, W6L, and W7 (Figure 10b; Sun et al., 2018). At present, only W1L, W2L, W3L, and W4L glauberite seams are used as brine sources. There are two artesian brine aquifers, W2X and W3X, exposed by drill holes in the Xinqing platform and there are three beds in the Tenglong extraction area, including a phreatic aquifer, W1T, and two artesian aquifers, W2T and W3T (Figure 10b).

W1L is a phreatic aquifer with layered distribution across the whole Luobei depression, with an average thickness of 17.54 m, water table depths of 1.7 to 2.3 m, porosities of 6.98% to 38.45%, and specific yields of 4.57% to 25.89%. Water yield is the highest in the central and northeast of the depression, with unit brine overflows of more than 5000 cubic meters per day per meter of water table depth (m3/dm). In the rest of the aquifer, the unit brine overflows range from 1000 to 5000 m3/dm (Sun et al., 2018). The W2L artesian aquifer is confined, nearly horizontal with a stratified distribution, and has an average thickness of 10.18 m, unit brine overflows of 10 to 100 m3/dm, water table depths of 20 to 40 m, porosities of 4.34% to 37.8%, and specific yields of 1.08% to 21.04%. The W3L artesian aquifer is confined, with stratified distribution and an average thickness of 8.50 m, unit brine overflows of 10 to 100 m3/dm, water table depths of 40 to 70 m, porosities of 2.85% to 19.97%, and specific yields of 1.10% to 13.37%. The W3L aquifer is also confined with stratified distribution, with an average thickness of 7.28 m, unit brine overflows of 10 to 100 m3/dm, water table depths of 70 to 100 m, porosities of 5.22% to 24.72%, and specific yields of 1.03% to 9.91%. The lithologies of the four brine storage layers are dominated by glauberite, and occasional lacustrine sedimentary clastic rocks, such as gypsum (Figure 10a).

The Xinqing platform consists of two confined potassium-bearing brine aquifers (Figure 10b). Confined brines have layered or stratified distributions. The average thicknesses of the aquifers are 4.38 to 7.52 m. Due to the F1 fault, there is no phreatic aquifer in the Xinqing platform, but this does not affect the continuity of the brine storage layer between the extraction areas. The W2X aquifer is confined, stratified, and distributed in the eastern part of this ore district with a north-south length of 77.78 km, east-west width of 16.82 km, and total area of 1100 km2. Unit brine overflows are 2.25 to 541.51 m3/dm, water table depths are 10 to 20 m, porosities are 3.89% to 40.69%, and specific yields are 2.01% to 21.15%. The W3X aquifer is also confined and stratified, with a north-south length of 76.10 km, east-west width of 18.81 km, and total area of 1444 km2. Unit brine overflows are 1.67 to 293.99 m3/d m, water table depths are 11.3 to 38 m, porosities are 4.16% to 26.43%, and specific yields are 2.11% to 14.19%23.

The Tenglong platform consists of a phreatic aquifer and two confined aquifers. W1T is a phreatic, stratified aquifer and is the main ore body, and is bound by the F3 fault (Figure 10b). It is distributed across the northern part of the Tenglong extraction area, with a north-south length of about 33 km, east-west width of about 20 km, and total area of 610 km2. Water table depths are 3.26 to 4.6 m, porosities are 2.03% to 38.81%, and specific yields are 22.48% to 1.22%. On the other side of the F3 fault, in the southern part of the mining area, is the W2T confined aquifer (Figure 10b). Water table depths are 16.91 to 22 m, porosities are 3.58% to 37.64%, and specific yields are 1.35% to 18.69%. W3T is also a confined aquifer, with a stratified orebody distributed in the southern part of the mining area, with a north-south length of about 29 km, east-west width of about 21 km, and total area of 546 km2. Water table depths are 17.13 to 47 m, porosities are 2.69% to 38.71%, and specific yields are 1.26% to 17.64%.

Lop Nur is an unusual potash source

The glauberite-hosted brinefield in the Luobei depression and the adjacent platforms makes the Lop Nur SOP system unique in that it is the world's first large-scale example of brine commercialisation for potash recovery in a Quaternary continental playa aquifer system with a non-MOP brinefield target. Elsewhere, such as in the Dead Sea and the Qarhan sump, Salar de Atacama and the Bonneville salt flats, the brines derived from Quaternary lacustrine beds and water bodies are concentrated via solar evaporation in semi-arid desert scenarios. Potash plants utilising these Quaternary evaporite-hosted lacustrine brine systems do not target potassium sulphate, but process either carnallitite or sylvinite into a commercial MOP product

 Glauberite is found in a range of other continental Quaternary evaporite deposits around the world but, as yet,                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       outside of Lop Nur is not economically exploited to produce sulphate of potash. For example, glauberite is a significant component in Quaternary cryogenic beds in Karabogazgol on the eastern shore of the Caspian Sea, in Quaternary evaporite beds in Laguna del Rey in Mexico, in saline lacustrine beds in the Miocene of Spain and Turkey, and in pedogenic beds in hyperarid nitrate-rich soils of the Atacama Desert of South America (Warren, 2016; Chapter 12).

In most cases, the deposits are commercially exploited as a source of sodium sulphate (salt cake). In a saline Quaternary lake in Canada, SOP is produced by processing saline lake waters. This takes place in Quill Lake, where small volumes of SOP are produced via mixing a sylvite feed (trucked into the site) with a cryogenic NaSO4 lake brine.

The Lop Nur deposit is mined by the SDIC Xinjiang Luobupo Hoevellite Co. Ltd, and the main product is potassium sulfate, with a current annual production capacity of 1.3 million tons. Pan construction began in 2000, and the plant moved in full -cale operation in 2004 when it produced ≈50,000 tons. The parent company, State Development and Investment Corporation (SDIC), is China’s largest state-owned investment holding company. The company estimates a potash reserve ≈ 12.2 billion tons in the sump. This makes Lop Nur deposit the largest SOP facility in the world, and it is now a significant supplier of high premium fertiliser to the Chinese domestic market.

Implications

A study of a few of the Quaternary pans worldwide manufacturing economic levels of potash via solar evaporation shows tha,t independent of whether SOP or MOP salts are the main product, all retain abundant evidence that salt precipitates continue to evolve as the temperature and the encasing brine chemistry change. As we shall see in many ancient examples discussed in the next article, ongoing postdepositional mineralogical alteration dominates the textural and mineralogical story in most ancient potash deposits.

As we saw in the previous article, which focused on MOP in solar concentrator plants with brine feeds from Quaternary saline lakes, SOP production from brine feeds in Quaternary saline lakes is also related strongly to cooler desert climates (Figure 12). The Koeppen climate at Lop Nur is cool arid desert (BWk), while the Great Salt Lake straddles cool arid steppe desert and a temperate climate zone, with hot dry summer zones (BSk and Csa)


Outside of these two examples, there are a number of other Quaternary potash mineral occurrences with the potential for SOP production, if a suitable brine processing stream can be devised (Warren, 2010, 2016). These sites include intermontane depressions in the high Andes in what is a high altitude polar tundra setting (Koeppen ET), none of which are commercial (Figure 12b).

Similarly, there a number of non-commercial potash (SOP) mineral and brine occurrences in various hot arid desert regions in Australia, northern Africa and the Middle East (Koeppen BWh). Today, SOP in Salar de Atacama is currently produced as a byproduct of lithium carbonate production, along with MOP, as discussed in the previous article in this series.

As for MOP, climatically, commercial potash brine SOP systems are hosted in Quaternary-age lacustrine sediments are located in cooler endorheic intermontane depressions (BWk, BSk). The association with somewhat cooler desert and less arid cool steppe climates underlines the need for greater volumes of brine to reside in the landscape in order to facilitate the production of significant volumes of potash bittern.

Put simply, in the case of both MOP and SOP production in Quaternary settings, hot arid continental deserts simply do not have enough flowable water to produce economic volumes of a chemically-suitable mother brine. That is, currently economic Quaternary MOP and SOP operations produce by pumping nonmarine pore or saline lake brines into a set of concentrator pans. Mother waters reside in hypersaline perennial lakes in steep-sided valleys or in pores in salt-entraining aquifers with dissolving salt compositions supplying  a suitable ionic proportions in the mother brine. In terms of annual volume of product sold into the world market, Quaternary brine systems supply less than 15% The remainder comes from the mining of a variety of ancient solid-state potash sources. In the third and final article in this series, we shall discuss how and why the chemistry and hydrogeology of these ancient potash sources is mostly marine-fed and somewhat different from the continental hydrologies addressed so far.

References

Behrens, P., 2002, Industrial processing of Great Salt lake Brines by Great Salt Lake Minerals and Chemical Corporation, in D. T. Gywnne, ed., Great Salt Lake: A scientific, historical and economic overview, Utah Geological and Mineral Survey, Bulletin 116, p. 223-228.

Bingham, C. P., 1980, Solar production of potash from brines of the Bonneville Salt Flats, in J. W. Gwynn, ed., Great Salt Lake; a scientific, history and economic overview. , v. 116, Bulletin Utah Geological and Mineral Survey, p. 229-242.

Butts, D., 2002, Chemistry of Great Salt Lake Brines in Solar Ponds, in D. T. Gywnne, ed., Great Salt Lake: A scientific, historical and economic overview, Utah Geological and Mineral Survey, Bulletin 116, p. 170-174.

Butts, D., 2007, Chemicals from Brines, Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, Inc., p. 784-803.

Chao, L., P. Zicheng, Y. Dong, L. Weiguo, Z. Zhaofeng, H. Jianfeng, and C. Chenlin, 2009, A lacustrine record from Lop Nur, Xinjiang, China: Implications for paleoclimate change during Late Pleistocene: Journal of Asian Earth Sciences, v. 34, p. 38-45.

Dong, Z., P. Lv, G. Qian, X. Xia, Y. Zhao, and G. Mu, 2012, Research progress in China's Lop Nur: Earth-Science Reviews, v. 111, p. 142-153.

Felton, D., J. Waters, R. Moritz, D., and T. A. Lane, 2010, Producing Sulfate of Potash from Polyhalite with Cost Estimates, Gustavson Associates, p. 19.

Hu, G., and N.-a. Wang, 2001, The sand wedge and mirabilite of the last ice age and their paleoclimatic significance in Hexi Corridor: Chinese Geographical Science, v. 11, p. 80-86.

Huntington, E., 1907, Lop-Nor. A Chinese Lake. Part 1. The Unexplored Salt Desert of Lop: Bulletin of the American Geographical Society, v. 39, p. 65-77.

Jones, B., D. Naftz, R. Spencer, and C. Oviatt, 2009, Geochemical Evolution of Great Salt Lake, Utah, USA: Aquatic Geochemistry, v. 15, p. 95-121.

Lin, Y., L. Xu, and G. Mu, 2018, Differential erosion and the formation of layered yardangs in the Loulan region (Lop Nur), eastern Tarim Basin: Aeolian Research, v. 30, p. 41-47.

Liu, C., W. Mili, J. Pengcheng, L. I. Shude, and C. Yongzhi, 2006, Features and Formation Mechanism of Faults and Potash-forming Effect in the Lop Nur Salt Lake, Xinjiang, China: Acta Geologica Sinica - English Edition, v. 80, p. 936-943.

Liu, C.-A., H. Gong, Y. Shao, Z. Yang, L. Liu, and Y. Geng, 2016a, Recognition of salt crust types by means of PolSAR to reflect the fluctuation processes of an ancient lake in Lop Nur: Remote Sensing of Environment, v. 175, p. 148-157.

Liu, C. L., M. L. Wang, P. C. Jiao, W. D. Fan, Y. Z. Chen, Z. C. Yang, and J. G. Wang, 2008, Sedimentary characteristics and origin of polyhalite in Lop Nur Salt Lake,Xinjiang: Mineral Deposits.

Liu, C. L., J. F. Zhang, P. C. Jiao, and S. Mischke, 2016b, The Holocene history of Lop Nur and its palaeoclimate implications: Quaternary Science Reviews, v. 148, p. 163-175.

Ma, C., F. Wang, Q. Cao, X. Xia, S. Li, and X. Li, 2008, Climate and environment reconstruction during the Medieval Warm Period in Lop Nur of Xinjiang, China: Chinese Science Bulletin, v. 53, p. 3016-3027.

Ma, L., T. K. Lowenstein, B. Li, P. Jiang, C. Liu, J. Zhong, J. Sheng, H. Qiu, and H. Wu, 2010, Hydrochemical characteristics and brine evolution paths of Lop Nor Basin, Xinjiang Province, Western China: Applied Geochemistry, v. 25, p. 1770-1782.

Spencer, R. J., H. P. Eugster, and B. F. Jones, 1985b, Geochemistry of Great Salt Lake, Utah II: Pleistocene-Holocene evolution: Geochimica et Cosmochimica Acta, v. 49, p. 739-747.

Spencer, R. J., H. P. Eugster, B. F. Jones, and S. L. Rettig, 1985a, Geochemistry of Great Salt Lake, Utah I: Hydrochemistry since 1850: Geochimica et Cosmochimica Acta, v. 49, p. 727-737.

Sun, M.-g., and L.-c. Ma, 2018, Potassium-rich brine deposit in Lop Nor basin, Xinjiang, China: Scientific Reports, v. 8, p. 7676.

Warren, J. K., 2010, Evaporites through time: Tectonic, climatic and eustatic controls in marine and nonmarine deposits: Earth-Science Reviews, v. 98, p. 217-268.

Warren, J. K., 2016, Evaporites: A compendium (ISBN 978-3-319-13511-3): Berlin, Springer, 1854 p.

 

Danakhil Potash; Ethiopia - Modern hydrothermal and deep meteoric KCl, Part 3 of 4

John Warren - Friday, May 01, 2015

So far we have discussed the modern salt pan geology of the Danakhil (Part 1 of 4) and the initial subaqueous setting for widespread bedded potash, now in the subsurface, mostly as a kainitite bed (Part 2 of 4). In this blog we will discuss examples of potash in the Danakhil where remobilised salts and brines are related to the circulation of hydrothermal and meteoric fluids have facilitated localised reworking of potash to the surface (part 3 of 4). These fluids are related to the thermal anomalies created by the emplacement of the Dallol mound and the chemical front created by the encroachment of the Bajada along the western margin of the saltflat. Notably, we shall see the Dallol Mound is not a volcanic cone, rather it is an anticlinal dome of uplifted and eroded bedded salt, capped and surrounded by hydrothermal crater features typified by karst pools and brine outflows. Its creation is likely related to emplacement of igneous material at depth but, as yet there, has been no breakout of volcanic rock material in the mound area. This has important economic implications for the nature of remobilised potash and the creation of potential potash ores in the Dallol Mound area, these cosiderations are separate from the regional distribution of primary potash beds (kainitite and carnallitite) that were discussed in the previous blog.


Thermal brine springs and potash occurrences near Dallol mound

Today, hot springs supply and maintain a number of hydrothermally-fed brine pools and brine filled karst lakes in various depressions both atop and near the regional anticlinal salt mound or salt dome, sometimes called Dallol Mountain (Figure 1). As it only rises some 60 metres from the surrounding surface (-81 m versus -120 m) the term mountain is a misnomer. The highly dissected and eroded slope of bedded halite that is the southwest margin of Dallol mound shows the various springs are active in a region of uplifted and eroded bedded evaporite that defines the Dallol mound (Figure 2a). For example, brine springs still supply a small carnallite deposit known as the Crescent deposit located near the uplifted black halite beds that define Black Mountain and located 1.5 km southwest of Dallol mound (Figure 2b). This potash ore is the result of hydrothermally-driven groundwater activity, likely related to the emplacement of the Dallol Mound. The uplift-related thermal hydrology has broken up the mineralogical continuity of the nearsurface evaporite beds including the equivalents to the potash-rich Houston Fm.


The Black Mountain potash deposits caught the attention of the Houston-based  Ralph M. Parsons company in 1954 where, according to Holwerda and Hutchinson, 1968, potash mining had previously already taken place at the Crescent carnallite/sylvite deposit. Earlier extraction had involved, amongst other techniques, flooding of salt pans around a continuously flowing hot spring, followed by harvesting of potash-rich salts, once natural deliquescence had flushed most of the highly soluble MgCl2 from the system. A concession was obtained Parsons linked to obligations to investigate the various potash deposits in the area, some of which were tied to actual outcrops of potash salts. The Parsons Company set up its base on Dallol Mountain at a site previously occupied by the Italian mining community, which had operated in the first few decades of last century (Figure 2a; the modification and reuse of older salt brick buildings is still evident on the ground today). As well, Parsons Co. constructed airstrips on Dallol Mountain and in the Musley area. They drilled more than 300 holes in order to better understand the the distribution of the potash beds. Drilling operations in 1959-1961 led to the delineation of the small localized "Crescent" carnallitite deposit in the vicinity of Black Mountain . This was followed by the discovery of the much larger (>80 million tonnes) "Musley" sylvite deposit near the base of the Ethiopian Highlands, some 5km W of Dallol, and extending at least 10km in a N-S orientation. A 92m vertical shaft and a total of 805m of drives were made in this deposit, but all work was stopped in 1967 after rapid influx of water into the conventional mine killed a number of workers. The political tensions in the area at the time probably also played a part in preventing mining activity in the following years.

Holwerda and Hutchinson (1968) argue that geographical location of the main "Musley" sylvite strata is directly west of Dallol Mound and at the base of the highlands. This, and the fact that sylvite is an alternation product that consistently overlays the carnallite strata and thickens (although discontinuously) along the western margin (see drill hole intersections published in Ercosplan, 2011), suggests that the potash enrichment was produced by selective leaching of MgCl2 from a carnallite precursor, driven by phreatic run-off waters sourced in the Ethiopian highlands. My own observations and plotting of enrichment fairways (using published Ercosplan 2010, 2011 data) confirms Holwerda and Hutchinson’s inferences. If diagenesis, not primary precipitation, is the prime mechanism of sylvite creation in the Musley region, then the regional sylvite control/distribution for this style of enrichment is related to a subsurface meteoric/groundwater phreatic overprint that parallels the encroaching bajada edge. It is a separate ore fairway to the more regional easterly dipping bedded kainitite/carnallitite trend.

Waters in some of the active brine-filled hydrothermal craters and dolines can locally have temperatures of more than 100°C and when waters cool they precipitate varying combinations of halite, carnallite and bischofite. The brines are so saturated with salts that if a stick is thrust into a boiling brine pool and removed it is immediately covered by layer of carnallite or bischofite and halite (Figure 2b, c). The same pools are also rich in FeCl2, sulphur and manganese, which explains the spectacular bright green, red-orange and yellow colours of many of the saline mineral assemblages precipitating in and about these active spring-formed pools. Occasional intense storm-driven sheetfloods can drive renewed activity in the various springs in vicinity of the mound, as happened in the recent floods of February 2011, when the intensity of water circulation and the areal extent of the pools greatly increased. After the same storm flood, a natural collapse doline tens of metres across formed on the western depression margin. Clearly, the local hydrothermal/karstic enhancement style of bittern enrichment is a separate process set for potash enrichment compared to the widespread earlier deposition of marine-fed subaqueous kainite. Hence, it contrasts with the much more widespread set of depositional/early diagenetic processes that laid down the bulk of the bedded potash association that is the Houston Fm. in the Danakhil Depression (as discussed in the previous Danakhil blog).

What is the Dallol Mound and what drives its uplift hydrology?

Despite the widespread misconception that the Dallol mound is a lava cone, Mount Dallol is not a volcanic-centered feature on the Danakhil landscape. A visit to the area reveals no observable volcanic products (lava, ashfall or scoria) on the surface on or near the Dallol mound. This is so even in the region of the most recent phreatic activity in 1926 where a 30 m-diameter phreatic (explosion? or daylighting hydrothermal karst) crater formed, hosted in salt beds (Figure 2b). All the rocks associated with this cavity and its formative event are not volcanic. This means the mechanism that created the Dallol Mound is unlike the magmatic events that created the world famous Erte Ale volcanic cone, with its distinctive longterm active magma lake and located some 80 km to the south of Dallol and still in the Danakhil depression. Instead, the Dallol mound crest is made up of uplifted and eroded halite and potash beds soaked in a thermal hydrology that breaks out on the lake surface as a number of hot bubbling sulphurous brine pools. This is also true of the off-mound crater that formed in 1926 near Black Mountain and still retains bubbling brines with present temperatures ~65-70 °C. Nearby “Black Mountain” is a small area of dark coloured bedded and recrystallised halite, it is not a primary volcanic feature.

As a sedimentologist visiting the area, I wondered at why the Dallol mound features had ever been called volcanic cones, hornitos, or maars (as they are widely described in the literature). To use such genetic terms in a geologically correct fashion I would like to put my hand on a piece of volcanic debris (lava, pumice, scoria or ash) in any of the craters before I call the Dallol mound a volcanic cone. And yet, many workers in the published literature dealing with the Dallol area are happy to do this. I am not saying there is no influence of magmatic heating in forming Dallol Mound, only that molten volcanic rock has yet to surface in the immediate Dallol region. Hence it is unlike the many actual volcanic cones, maars and hornitos to the south and north and this is an significant observation as it deals with mechanism of local potash enrichment. I will argue in the next section that this is because Dallol Mound is a salt uplift feature or dome capped by phreatic cone/ hydrothermal karst structures and all related to the migration of molten magma into more deeply buried salt beds, which contain hydrated salts at the level of the Houston Fm and perhaps even deeper buried hydrated salt layers (see blog 2).

Darrah et al (2013) and Detay (2011) argue that the 30m diameter 1926 crater and other nearby pools on the Dallol saltflat in the vicinity of the Dallol mound are the result of a phreatic explosions, tied to the increasing gas pressure in superficial hydrothermal reservoirs atop a deeper mass of molten rock. The mound is a landscape feature indicative of deep dyke/sill intrusion that did not surface. According to Holwerda and Hutchinson (1968) this yet-to-daylight dyke complex explains the linear orientation of the mound, its pools and other karst/erosion features on the salt flat surface in vicinity of the Dallol mound. That is, the various Dallol hot springs typically consist of 30-40m diameter circular to sub-circular ponds, initially formed by explosive vapor eruptions, to form at-surface circular features, which are widely termed maars, although I would prefer to call them "maar-like." A “maar” is defined in the AGI Glossary of Geology as “a low relief, broad volcanic crater formed by multiple shallow explosive eruptions. It is surrounded by a crater ring, and may be filled by water. Type occurrence is in the Eifel area of Germany.” Given the lack of a volcanic crater rim the Dallol Mound and adjacent brine-filled cavities are not really maars, nor are they hornitos. They will likely evolve into such features, but in their current state better considered brine-filled fumaroles or solfateras or even better, hydrothermal karst cavities that have daylighted. Once the cavities have broken out onto the salt flat surface, these circular (possibly-explosive) features can continue enlarge due to ongoing rise of undersaturated waters and so evolve into expanding hydrothermal karst pools or they can be partially to completely filled with saline precipitates (with no volcanic products derived from molten igneous rock materials).


So, instead of at-surface volcanic products such as lava and ashfall, most of the superficial precipitates/sediments observed in and around the various on- and off-structure Dallol brine pools are evaporite salts, along with some remnants of older clay-sediments. Brine fluids in various hot spring pools in the Dallol area (in the Dallol “hill” crest and the “Crescent” region near Black mountain, and in the “Boiling Lake” region south of the mound) are typically multi-coloured warm/hot ponds (Figure1, 3; Gebresilassie et al., 2011). The various pools are extremely salty (>500g/L), can be highly acidic (sometimes with a pH approaching 0.5), and gas-rich (as evidenced by steady, vigorous bubbling of gases). According to Darrah et al. (2013) the Dallol “salt dome” fluids and associated hot springs are hypothesized to result from the interaction between hot mantle fluids or basalt dyke injections with evaporite deposits at unknown depths. However, direct observations of the volumes of pool waters and the vigour of the outflow are known to increase after the occasional heavy rain event, as happened in February, 2011. Hence, it is unclear if sulfur-rich gases and the low pH brine fluids provide evidence of the interaction of hot mantle fluids with the evaporites (as inferred by Darrah et al., 2013) or the pool waters are, at least in part, related shallower ongoing hydrothermal/karst interactions with more deeply circulated meteoric waters sourced in the 1000-m high adjacent rift highlands.

Why hydrated salts are important in some salt-hosted thermal systems: a Permian Zechstein analog

Most published volcanogenic-related studies of the Dallol Mound have not considered the effects of hydrated salt layers in a situation of rising molten rock, where the country rock contains beds of hydrated evaporites such as kainite or carnallite. This situation is exposed in the dyke-intruded halite-carnallite levels in the mines of the Werra-Fulda mine district of Germany (Schoefield et al., 2013; Warren, 2015). There, the Permian Zechstein salt series contains two important potash salt horizons (2-10m thick), which are mined at a depths ≈ 800 m from within a 400m thick halite host (Figure 4a). In the later Tertiary, basaltic melts intruded these Zechstein evaporites, but only a few dykes reached the Miocene landsurface. Basaltic melt production was related to regional volcanic activity some 10 to 25 Ma. Basalts exposed in the mine walls are typically subvertical dykes, rather than sills. These basaltic intervals can crosscut the salt over zones up to several kilometres wide (Figure 4b). However, correlations of individual dyke swarms, between different mines, or between surface and subsurface outcrops, is difficult.


The basalts are phonolitic tephrites, limburgites, basanites and olivine nephelinites. Dyke margins in contact with halite are usually vitrified, forming a microlitic limburgite glass along dyke edges (Knipping, 1989). At the contact on the evaporite side of the glassy rim there is a cm-wide carapace of high temperature salts (mostly anhydrite and ferroan carbonates). Further out, the effect of the high temperature envelope is denoted by transitions to clear halite, with higher temperature fluid inclusions (Knipping 1989). All of this centimetre to metre-scale alteration is an anhydrous alteration halo, the salt did not melt (halite’s melting temperature is 804°C), rather than migrating, the fluid driving recrystallisation was largely from local movement of entrained brine inclusions. The dolerite/basalt interior of the basaltic dyke is likewise altered and salt soaked, with clear, largely inclusion-free halite typically filling vesicles in the basalt.

Worldwide, dykes intersecting salt beds tend to widen to become sills in two zones: 1) along evaporite units within the halite mass that contain hydrated salts, such as carnallite or gypsum and, 2) where rising magma has ponded and so created laccoliths at the upper or lower halite contact with the adjacent nonsalt strata or against a salt wall (Warren, 2015). The first is a response to a pulse of released water as dyke-driven heating forces the dehydration of hydrated salt layers. The second is a response to the mechanical strength contrast at the salt-nonsalt contact. The first is what is observed in the Fulda region and is also likely relevant to the formation of the Dallol Mound and its remobilised potash-precipitating brines.

 

In such subsurface regions, the heating of hydrated salt layers (such as carnallite or kainite), adjacent to a dyke or sill, drives off the water of crystallisation (chemical or hydration thixotropy) at a much lower temperatures than that at which anhydrous salts, such as halite or anhydrite, thermally melt (Table 1). In the Fulda region the thermally-driven release of water of crystallisation within particular Zechstein salt beds creates thixotropic or subsurface “peperite” textures in carnallitite ore layers, where heated water of crystallisation escaped from the hydrated-salt lattice. Dehydration-driven loss of mechanical strength focuses zones of magma entry into particular horizons in the salt mass, wherever hydrated salt layers were intersected (Figure 4c verses 4d). In contrast, dyke and sill margins are much sharper and narrower in zones of contact with anhydrous salt intervals (Figure 4b; Schofield et al., 2014).

Accordingly, away from immediate vicinity of the direct thermal aureole, heated and overpressured dehydration waters can enter a former Zechstein carnallite halite bed, and drive the creation of extensive soft sediment deformation and [1]peperite textures in the former hydrated layer (Figure 4d, e). Mineralogically, sylvite and coarse recrystallised halite dominate the salt fraction in the peperite intervals/beds. These are evaporite-related beds formed within a hydrated salt bed and so differ from the common notion of volcanic peperites indicating water-saturated sediment intercations with very shallow dyke or sill emplacements. Sylvite in these altered zone is a form of dehydrated carnallite, not a primary-textured salt. In the Fulda region, such altered zones and deformed units can extend along former carnallite layers to tens or even a hundred or more metres from the dyke feeder. Ultimately, the deformed potash bed passes laterally out into the unaltered bed, which retains abundant inclusion-rich primary chevron halite and carnallite (Figure 4d versus 4e). That is, nearer the basalt dyke, the carnallite is largely transformed into inclusion-poor halite and sylvite, the result of incongruent flushing of warm saline fluids mobilized from the hydrated carnallite crystal lattice as it was heated by dyke emplacement. During Miocene salt alteration/thermal metamorphism in the Fulda region, NaCl-fluids were mixed with fluids originating from thermally-mobilised crystallisation water in the carnallite, as it converted to sylvite.

In the Dallol depression I think it is highly likely that a similar set of destabilisation processes occurred when rising dyke magma reached the levels of hydrated salts (kainite and carnallite beds) in the Houston Formation of the Danakhil fill, after passing relatively passively through the Lower Rocksalt Formation (see the previous blog). Emplacement of the magma/dyke into  hydrated evaporites in the vicinity of what is now the Dallol mound would have mobilised and deformed the hydrated salt level, converting carnallite to sylvite, kainite to bischofite and lesser kieserite, as well as creating widespread cavities filled with pressured volatiles carried by MgCl and KCl brines. Once these hydrothermal cavities dissolve their way to surface, the feeder brines can cool and precipitate as prograde salts such as halite, sylvite and perhaps bischofite. Such destabilisation would have accommodated the emplacement of a basaltic sill at the level of the potash salts, in turn driving the uplift of the lake beds above this region. Mound-related uplift and hydrothermal activity then drives the formation of natural regions of ground collapse, sulphurous and acidic springs and fumaroles, along with the creation of water-filled chimneys and doline sags, filling with various hydrothermal salts, in the vicinity of the volcanic mound.

Implications for Potash distribution in the Danakhil Depression

The discussion of potash mineral-forming processes in this and the previous blog clearly underlines a trichotomy in the way potash has accumulated in halite host-beds across the Danakhil Depression. The most widespread form of potash in the Danakhil Depression is as a primary evaporite bed, composed of primary marine kainitite precipitates with a carnallite cap (Houston Formation). Across the western side of the depression this easterly dipping bed is now buried beneath 30-150 m of overburden salts. It likely precipitated as a marine seepage-fed bittern layer, at a time the Danakhil depression was hydrographically isolated from a direct surface connection with the Red Sea. Its brine hydrology was dominantly subaqueous and not unlike that of modern Lake Asal in Djibouti, although it was more saline than Asal in the subaqueous potash sump areas. Thus, the Danakhil potash bed (Houston Fm) formed sometime ago, its formative hydology is no longer present in the depression and it may be as old as Pliocene or more likely early to mid Pleistocene. There has been sufficient time for this bed to tilt toward the east. The unit is underlain by the subaqueous Lower Rocksalt Formation (LRF) and subsequently overlain by the Upper Rocksalt Formation (URF). Both these halite formations do not entrain primary potash beds. The LRF contains numerous CaSO4 layers, while the URF contains clayey laminite beds and locally hosts regions of remobilised potash salts. The URF evolves upward into the saltflat/ephemeral lake hyperarid hydrology that typifies the modern depression.

More localised forms of potential potash ore typify occurrences in the Dallol and Musley areas (Figure 2a). There potash in the Dallol Mound region is hydrothermally reworked from the uplifted equivalents of the Houston Formation. Even today this hydrology is precipitating carnallitite (associated with bischofite and minor kieserite) in various hydrothermal brine pools atop and around the Dallol Mound, such as the carnallite-dominant Crescent deposit (Figure 2b). These hydrothermal salts owes their origins to daylighting of pressurised fluid systems and cavities. They were created by the volatile products of hydrated salt layers (Houston Fm) where these salts had come into contact with thermal aureoles or actual lithologies of newly emplaced dykes that had penetrated the underlying halite section. Actual molten volcanic rock has yet to make it to the surface in the Dallol Mound region, although active volcanic mounds and flows do typify the saltflat surface tens of kilometres to the south (Erte Alle ) and north. Based on the analogy exposed within the Zechstein-hosted potash mines of the Fulda region of Germany, it is likely that as well as creating at-surface brine pools, this hydrothermal dyke-related hydrology converts any carnallitite to a sylvinite bed at the level of contact with the Houston Fm. 

Then there is the deep-meteoric alteration system that is altering the kainitite/carnallitite of Houston Fm into sylvinite, it is active along the deep meteoric alteration front located at the irregular interface between the downdip end of the Musley Fan and the updip portion of the Houston Fm. This diagenetic mechanism formed the Musley potash deposit, defined and exploited by the Parsons Company operations and documented in Holwerda and Hutchison (1968). Variations on this deep-meteoric alteration theme likely extend south and north of the Musley fan, wherever the active phreatic hydrology of the bajada located at the foot of the Ethiopian Highlands interacts and interfingers with the updip edge of the easterly dipping Houston Formation.

Once again there is no "one-size-fits-all) model for economic potash understanding (Warren, 2010, 2015). Even in what is probably the youngest known marine-fed potash system in the world, the original potash mineralogy and distribution has been altered and locally upgraded via diagenetic interactions with hydrothermal or deep-meteoric fluids. Predicting ore distributions in this, and all potash systems worldwide, requires an understanding of formative process evolution through deep time, and not just the simple application of a layer-cake primary stratigraphic model. 

References

Carniel, R., E. M. Jolis, and J. Jones, 2010, A geophysical multi-parametric analysis of hydrothermal activity at Dallol, Ethiopia: Journal of African Earth Sciences, v. 58, p. 812-819.

Darrah, T. H., D. Tedesco, F. Tassi, O. Vaselli, E. Cuoco, and R. J. Poreda, 2013, Gas chemistry of the Dallol region of the Danakil Depression in the Afar region of the northern-most East African Rift: Chemical Geology, v. 339, p. 16-29.

Detay, M., 2011, Le DALLOL revisité: entre explosion phréatomagmatique, rifting intra-continental, manifestations hydrothermales et halocinèse: LAVE. Liaison des amateurs de volcanologie européenne, v. 151, p. 7-19.

ERCOSPLAN, 2010, Techical report and current resource estimate: Danakhil Potash Deposit, Afar State, Ethiopia: Project Reference: EGB 08-024.

ERCOSPLAN, 2011, Preliminary Resource Assessment Study, Danakhil Potash Deposit, Afar State, Ethiopia: G & B Property: Project Reference: EGB 10-030.

Gebresilassie, S., H. Tsegab, and K. Kabeto, 2011, Preliminary study on geology, mineral potential, and characteristics of hot springs from Dallol area, Afar rift, northeastern Ethiopia: implications for natural resource exploration: Momona Ethiopian Journal of Science, v. 3, p. 17-30.

Holwerda, J. G., and R. W. Hutchinson, 1968, Potash-bearing evaporites in the Danakil area, Ethiopia: Economic Geology, v. 63, p. 124-150.

Knipping, B., 1989, Basalt intrusions in evaporites: Lecture Notes in Earth Sciences (Springer-Verlag), v. 24, p. 132 pp.

Schofield, N., I. Alsop, J. Warren, J. R. Underhill, R. Lehné, W. Beer, and V. Lukas, 2014, Mobilizing salt: Magma-salt interactions: Geology.

Warren, J. K., 2010, Evaporites through time: Tectonic, climatic and eustatic controls in marine and nonmarine deposits: Earth-Science Reviews, v. 98, p. 217-268.

Warren, J. K., 2015, Evaporites: A compendium (ISBN 978-3-319-13511-3) Released August 2015: Berlin, Springer, 1600 p.

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[1] Peperite is a sedimentary rock that contains fragments of igneous material and is formed when magma comes into contact with wet water-saturated sediments. 

Danakhil Potash, Ethiopia: Beds of Kainite/Carnallite, Part 2 of 4

John Warren - Wednesday, April 29, 2015

The modern Dallol saltflat described in the previous blog defines the upper part of more than 970 metres of halite-dominated Quaternary evaporites that have accumulated beneath the present salt pan of the Northern Danakhil. The total sequence is made up of interbeds of halite, gypsum, anhydrite and shale with a potash succession separating two thick sequences of halite (Figure 1; Holwerda and Hutchison, 1968; Augustithis, 1980). At depths of more than 35-40 meters, and deepening to the east, this km-thick subcropping Quaternary halite-dominated fill contains one, and perhaps two or more, potash beds. For a more detailed description of the upper part of the fill the reader is referred to the previous blog and Chapter 11 in Warren, 2015.


Bedded Pleistocene evaporites may underlie the entire Danakil depression, but younger lava flows of the Aden Volcanic Series and alluvium washed in from the surrounding bajada obscure much of the older Pleistocene sedimentary series across much of the southern part of the depression beyond Lake Assale). Potash exploration drilling and core recovery is concentrated in the accessible parts of the northern Danakhil rift, where the saltflat facilitates vehicle access, compared with the lava-covered regions south of Lake Assale. The most recent volcanic activity affecting the known potash region was the emplacement of the Dallol Mound, which has driven local uplift of the otherwise subsurface potash section to where it approaches the surface in the immediate vicinity of the mound (Figure 2a).

Away from the Dallol volcanic mound the upper potash bed beneath the saltflat lies at a depth of 38-190 metres. A lower inferred potash bed likely occurs at depth along the eastern end of the saltflat, but this second bed is inferred from high API kicks in gamma logs run in deeper wells, no solid salt was recovered (Holwerda and Hutchison, 1968). The upper proven potash bed is now the target zone for a number of minerals companies currently exploring for potash in the region. Regionally, both potash units dip east, with the deepest indicators of the two units encountered by the drill in a single well on the eastern side of the saltflat at depths of 683 and 930 m, respectively (Figure 2: Holwerda and Hutchison, 1968). The likely Quaternary age of the potash units, the marine brine source, explains the high magnesium content of the potash bittern salts, as modern seawater contains high levels of Mg and SO4.


My study of core that intersected the potash interval and that is sandwiched between the Lower and Upper Halite units shows both the lower and the upper halite units retain pristine sedimentary textures, with features and vertical successions that indicate distinct hydrologies during their deposition (Figure 3). There is no textural evidence of halokinetic recrystallization in halites any of the studied cores and published seismic also indicates consistent dips in the evaporites . Most of the textures in the cored potash interval indicate a subaqueous density-stratified environment, with brine reworking of the upper part of primary kainitite, carnallitite units. Perennial subaqueous, density-stratified brines also typify the hydrology of the Lower Halite unit, albeit with somewhat lower salinities tan those precipitating the bitterns (Figure 3). The brine that precipitating the Upper Rocksalt Formation was shallower and more ephemeral. The following paragraphs summarise my core-based observations and interpretations that led to this interpretation of the evolving brine hydrology.

The Lower Rocksalt Formation (LRF) is dominated by bottom-growth-aligned subaqueous halite textures and lack of siliciclastic detritus, unlike the Upper Rocksalt Formation (Figure 3). Halite textures in the LRF lack porosity and dominated by coarsely crystalline beds made up of cm-scale NaCl-CaSO4 couplets dominated by upward-pointing halite chevrons and mantled by thin CaSO4 layers (Figure 3). This meromictic-holomictic textural association passes up into the upper part of the LRF with cm-scale proportions alternating of less-saline to more-saline episodes of evaporite precipitation decreasing, indicating an “on-average” increasingly shallow subaqueous depositional setting as one approaches the base of the kainitite unit. The combination of bottom-nucleated and cumulate textures in the LRF are near identical to those in the halites in the kainitite interval in the Messinian of Sicily (see later). 

The laminated Kainitite Member is also a subaqueous unit with layered cumulate textures (Figure 3), it was likely deposited on a pelagic bottom beneath a shallow body of marine-fed bittern waters, which never reached carnallite saturation. Above this are the variably present carnallitic Intermediate and Sylvinitite members and the overlying Halite marker beds in turn overlain by the Upper Halite unit. All retain pristine textures indicating mostly subaqueous deposition, soon followed by varying exposure and reaction with shallow phreatic brines moving across the top of Kainitite member. This shallow phreatic brine crossflow drove syndepositional mineral alteration and collapse in the upper part of the kainitite and carnallitite units.


The potash-entraining interval between the URF and LRF is called the Houston Formation has been drilled and cored extensively by explorers in the basin, showing it is consistently between 15 and 40 metres thick (Figure 1). Stratigraphically, it consists of lower Kainitite Member (4-14m thick) atop and in depositional continuity with the LRF (more than 500m thick) (Figure 3). The Kainitite Member is fine-grained, laminated, locally wavy-bedded, containing up to 50% kainite cumulates in a cumulate (non-chevron) halite background, along with small amounts of a white mineral that is likely epsomite. It is overlain by what older literature describes as the “Carnallitite Intermediate unit” (3-25 m thick). More recent potash exploration drilling has shown all the members that constitute the Intermediate Carnallitite Member is not always present within the Dallol depression. Mineralogically is at best considered as variably developed (Figure 3). Its lower part is a layered to laminated carnallite-halite mixture with some kieserite, anhydrite and epsomite. This can pass up or laterally into kainitite with sylvite. Above the Intermediate Member is the 0-10m thick Sylvinite Member containing 20-30% sylvite, along with polyhalite and anhydrite (up to 10%). Typically the sylvinite member shows primary layering disturbed by varying intensities of slumping and dissolution. Often the upper part of a carnallite unit (where present) also shows similar evidence of dissolution and reprecipitation.

Cores through the sylvinite member and parts of the upper carnallitite member sample a range of recrystallization/flow/slump textures, rather than primary horizontal-laminar textures. Beneath the sylvinite member, the variably-present upper carnallitite member contains a varied suite of non-commercial potash minerals that in addition to carnallite include, kieserite, kainite (up to 10% by volume) and polyhalite, along with minor amounts of sylvite. Minor anhydrite is common, while rinneite may occur locally, along with rust-red iron staining. Sylvite is more abundant near the top of the carnallitite member and its proportion decreases downward, perhaps reflecting its groundwater origin. Kainite is the reverse and its proportion increases downward. The sylvinite member and the carnallitite member also show an inverse thickness relationship. Bedding in the carnallitite member is commonly contorted with folded and brecciated horizons interpreted as slumps. The base of the carnallitite member is defined as the level where carnallite forms isolated patches in the kainite before disappearing entirely.

Drilling in the past few years has clearly show that in some parts of the evaporite unit, located nearer to the western side of the basin, the lower and upper carnallite units are separated by thick bischoftite intervals (Figures 2b, 3). The bischofite is layered at a mm-cm scale and with no obvious breaks related to freshening and exposure, implying it too was deposited in a perennially subaqueous or phreatic cavity setting (Pedley et al., in press).

The potash/bischofite interval passes up into a slumped and disturbed halite-dominated unit that is known as the “Marker Beds” because of the co-associated presence of clay lamina and bedded halite, along with traces of potash minerals (Figure 3). This unit then passes up into the massive Upper Rock Salt unit across an unconformity at the top of the halite “Marker Beds.” Bedded, and at times finely laminated cumulate textures in the various magnesian bittern units, are used by many to argue that the kainitite and the lower carnallitite members are primary or syndepositional precipitates.

Three types of potash-barren zones can occur within it and are possibly related to the effects of groundwaters and solution cavity cements within the carnallitite unit, perhaps precipitated before the deposition of the overlying marker halites. Barren zones in the Sylvinite member are regions where: a) the entire sylvinite bed is replaced by a relatively pure stratiform halite, along with dispersed nodules of anhydrite, b) zones up to 23 m thick and composed of pure crystalline halite (karst-fill cements?) that occur patchily within the sylvinite bed and, c) potash-depleted zones defined by coarsely crystalline halite instead of sylvinite. Bedding plane spacing and layering and some slumping styles in the halite in styles a and b are similar to that in the sylvite bed. Contact with throughflushing freshened nearsurface and at-surface waters perhaps created most of the barren zones in the sylvinite. Fluid crossflow may also have formed or reprecipitated sylvite of the upper member, via selective surface or nearsurface leaching of MgCl2 from its carnallite precursor (Holwerda and Hutchison, 1968; Warren 2015). Due to the secondary origin of much of the sylvite in the Sylvinite member, the proportion of sylvite decreases as the proportion of carnallite increases, along with secondary kieserite, polyhalite and kainite.

The kainite member is texturally distinctive and is composed of nearly pure, fine-grained, dense, relatively hard, amber-coloured kainite with ≈ 25% admixed halite (Figure 3). Core study shows the lamina style remained flat-laminar (that is, subaqueous density-stratified with periodic bottom freshening) as the mineralogy passes from the LRF up into the flat-laminated kainitite member (Figure 4: Warren, 2015; Pedley et al., in press). Throughout, the kainitite unit shows a cm-mm scale layering, with no evidence of microkarsting or any exposure of the kainitite depositional surface. That is, the Kainitite Member is a primary depositional unit, like the underlying halite and still retains pristine evidence of its dominantly subaqueous depositional hydrology. The decreased proportion of anhydrite in the Kainitite Member, compared to the underlying LRF, indicates a system that on-average was more saline than the brines that deposited the underlying halite. The preponderance of MgSO4 salts means the Kainitite unit like the underlying LRF formed by the evaporation of seep-supplied seawater.

This situation differs from the present “closed basin” hydrology of the Danakil Depression which typifies the URF and the overlying Holocene succession (Hardie, 1990; Warren, 2015).

Units atop the primary laminated textures of the kainitite, lower carnallitite and bischofite members (where present) tend to show various early-diagenetic secondary textures (Figure 4). It seems much of the sylvinite and upper carnallitite member deposition was in shallow subsurface or at-surface brine ponds subject to groundwater crossflows and floor collapse, possibly aided by seismically-induced pulses of brine crossflow. In addition, this perennial density-stratified brine hydrology was at times of holomixis subject to brine reflux and the brine-displacement backreactions that typify all evaporite deposition, past and present (Warren 2015).

The observation of early ionic mobility in potash zone brines in the Danakil depositional system is also not unusual in any modern or ancient potash deposit. It should not be considered necessarily detrimental to the possibility of an extensive economically exploitable potash zone being present in the Danakil Depression. Interestingly, all the world’s exploited potash deposits, including those in the Devonian of Canada and Belarus, the Perm of the Urals and the potash bed of west Texas, show evidence of syndepositional and shallow burial reworking of potash (Warren, 2015). Early potassium remobilization has created the ore distributions in these and other mined potash depositsTextures and mineralogies in the Upper Rocksalt Formation (URF) define a separate hydrological association to the marine-fed LRF and Houston Formation (Table 4). Compared to the LRF, the URF has much higher levels of depositional porosity, lacks high levels of CaSO4, and has high levels of detrital siliciclastics. This is especially so in its upper part, which shows textural evidence of periodic and ongoing clastic-rich sheetflooding and freshening (Figure 4). It was deposited in a hydrology that evolved up section to become very similar to that active on today’s halite pan surface. The URF contains no evidence of salinities or textures associated with a potash bittern event and is probably not a viable exploration target for solid potash salts.

Above the URF is a clastic unit with significant amounts of, and sometimes beds dominated by, lenticular gypsum and displacive halite. The unit thickens toward the margins of the depression (Figure 2). The widespread presence of diagenetic salts indicates high pore salinities as, or soon after, the saline beds that stack into the clastic unit were deposited. Some of these early diagenetic evaporite textures are spectacular, as seen in the displacive halite recovered in a core from the lower portion of the clastic overburden, some 45 m below the modern pan surface (Figure 3).

What is clear from the textures preserved in the potash-rich Houston formation and the immediately underlying and overlying halites is that they first formed in a subaqueous-dominated marine-fed hydrology (Figure 4), which evolves up section into more ephemeral saltpan hydrologies of today (see the previous blog). The potash interval encapsulated in the Houston formation has primary mineralogical associations that are derived by evaporation of Pleistocene seawater (kainitite, carnallitite). In contrast the sylvite section in the Houston tends to form when these primary mineralogies are altered diagenetically perhaps soon after deposition but, especially, when hydrothermal waters circulated through uplifted beds of the Houston Formation, as is still occurring in the vicinity of the Dallol Volcanic Mound. Or where the chemical/meteoric interface associated with the encroachment of the bajada sediment pile drove incongruent dissolution of carnallite along the updip edge of the Houston Fm (as we shall discuss in the next blog). 

References

Augustithis, S. S., 1980, On the textures and treatment of the sylvinite ore from the Danakili Depression, Salt Plain (Piano del Sale), Tigre, Ethiopia: Chemie der Erde, v. 39, p. 91-95.

Hardie, L. A., 1990, The roles of rifting and hydrothermal CaCl2 brines in the origin of potash evaporites: an hypothesis: American Journal of Science, v. 290, p. 43-106.

Holwerda, J. G., and R. W. Hutchinson, 1968, Potash-bearing evaporites in the Danakil area, Ethiopia: Economic Geology, v. 63, p. 124-150.

Pedley, H. M., J. Neubert, and J. K. Warren, in press, Potash deposits of Africa: African Mineral Deposits, 35TH International Geological Congress (IGC), Capetown (28 August to 4 September 2016).

Warren, J. K., 2015, Evaporites: A compendium (ISBN 978-3-319-13511-3) Released August 2015: Berlin, Springer, 1600 p. 


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